The percent of high severity defects is another good metric to be tracked. It takes ratio between issues with high severity (e.g., Blocker, Critical, Major), and all bugs that are raised for the evaluated period.
% of High Severity Defects = ( # Defect – Blocker, Critical, Major / # All Defects ) * 100

Check the following step by step KPI configuration using the Performance Objectives dashboard gadget:

Step 1: Add 2 data sources. One called ‘All issues’ with the complete data that is going to be analyzed and second data source called ‘High Severity Defects’ that contains only the Bugs with high severity/priority, e.g., Blocker, Critical and Major.

Step 2: Select ‘Formula’ for the metric type.
Step 3: Add 2 parameters for the Formula.
Parameter P1: with the number of issues from Data Source A: ‘All issues’.

Parameter P2: with the number of issues from Data Source B: ‘High Severity Defects’.

Step 4: Set the formula equation to (P2/P1)*100
Step 5: To show the data per month on the x-axis, switch the ‘Created’ date field granularity from ‘Week’ to ‘Month’ for the ‘Display by’ option.
Step 6: Group data by ‘Components’.
Step 7: Switch on Target section and keep ‘Fixed’ tab selected.
Step 8: Set results to be evaluated per x-axis item. In the sample case the target will be calculated against each component of the group and within each month shown on the x-axis.
Step 9: Set target as negative by selecting ‘Exceeding is’: Bad. This will reverse the RAG color scheme, and when the achieved values are above the Target, they will be considered as negative and will be colored in red.
Step 10: Set target value.
Step 11: Under the ‘More Settings’ section enable the ‘Warning threshold’. Thus, the values above the threshold and below the target level will be colored in amber.

Step 12: Under the ‘More Settings’ section enable the ‘Custom Target Label’ and set it to text that indicates the target is negative, e.g., Fail.
Recording: Please check the following recording with the detailed configuration settings:

To better understand incident severity levels, you may find this Atlassian article useful.
Explore our KPI Examples page for more performance indicators you can set and track on your dashboard with Performance Objectives for Jira app.